MEGA65 is a 100% open-source implementation of the official (but never-released) Commodore 65 computer. It is in development by associates of the Museum of Electronic Games and Art e. V., a not-for-profit institution "dedicated to the preservation of our digital heritage." As well as the original C65 design, the MEGA65 provides additional hardware and software enhancements, including a choice of using BASIC 10 or BASIC 65 (containing improvements that go beyond BASIC 10), and the MAC-like graphical user interface, GEOS.
The MEGA65 has an 8-bit CPU with additional 32-bit instructions implemented in FPGA. Like the original C65, it also has a Commodore 64 mode with a level of compatibility similar to that of the Commodore 128 running in C64 mode. It features HDMI and other modern improvements.
Source: C64-Wiki.